Hun, you shouldn't technically be on here at the age of twelve... But who really cares? I can't really talk, I was on here before I was of age.
And it's good to start by learning with a mouse, you get to know the programs better, and it's a good skill to have. If you get one in the future, it's good to be able to draw both with and without a tablet, just in case you need to get something down but you don't have it with you
Yes, In some ways, I think it's rather silly to have restrictions on registrations on this site. Art has no restrictions, and no age limit. But in other ways, I understand that they put it on here, because, well, this is the internet, and there is some mature content on this website. If you enjoy art and like to share it, then go for it!
It takes practice lol, you're not going to get something flawless on your first try, regardless of what program you use. The program is helpful but it won't do the job for you I'm willing to bet it took the artist a while to perfect this so it would be silly to expect instant perfection. Don't give up!